Traditionally, phantoms that closely mimic the physical properties of various human tissues have been very important for the development and testing of medical imaging modalities. Ideally, the material of the phantom should perfectly mirror the qualities of the bone being studied. For example, bone-mimicking phantom materials for use in ultrasound should have the same ranges of speeds of sound, attenuation coefficients, and backscatter coefficients as real bone. These parameters should be controllable in the manufacturing process of the phantom material, and their variation within the range of room temperatures should be small.
Speeds of sound in human bones vary over a fairly small range with an average value of about 3000 m/s. The amplitude of attenuation coefficients vary over the range from 2 dB/cm to about 4 dB/cm at a frequency of 1 MHz. The bones of the human skeleton can be divided into two types: cortical bone (around 80% of the total skeleton), and cancellous, or trabecular, bone (around 20% of the total skeleton). The latter has a porous structure made up of cortical trabecules, the pores being filled with bone marrow. Cortical bone has a homogenous, compact structure, being less than 10% porous. Trabecular bone has a much more complex composite structure.
The size, shape and concentration of pores vary between skeletal sites; the proportion by volume which is marrow is known as the porosity. Trabecular bone has a higher porosity, 50-90%, which makes its modulus and ultimate compressive strength around 20 times inferior than that of cortical bone. Hence, there is a requirement for the bone phantom material to be able to mimic porosity in a controllable way and allow it to imitate healthy and osteoporotic bones. Therefore, both the cortical and trabecular bone properties are very challenging to mimic, especially if in addition to ultrasound properties MRI compatibility is required, as in the case of HIFU therapy and anatomical MRI imaging methods. Making the ultrasound bone phantom material MRI compatible would considerably expand its applications for the development of ultrasound-based imaging diagnostic and therapeutic techniques. Currently there are no such ultrasound bone phantom materials available.